iio: maxim_thermocouple: detect invalid storage size in read()

As found by gcc -Wmaybe-uninitialized, having a storage_bytes value other
than 2 or 4 will result in undefined behavior:

drivers/iio/temperature/maxim_thermocouple.c: In function 'maxim_thermocouple_read':
drivers/iio/temperature/maxim_thermocouple.c:141:5: error: 'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]

This probably cannot happen, but returning -EINVAL here is appropriate
and makes gcc happy and the code more robust.

Fixes: 231147ee

 ("iio: maxim_thermocouple: Align 16 bit big endian value of raw reads")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
(cherry picked from commit 32cb7d27

)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>